In a supply network for supplying electrical power to electrical devices, line surge voltages may occur. In the following, such a supply network is also designated as a grid. Such line surge voltages are for example caused by a sudden release of energy that was previously stored in an energy storage device or where energy has been induced into the grid by other means, such as for example by inductive loads or lightning strikes. A line surge voltage typically occurs as a voltage pulse with a sharp rising edge and a comparably slow decay. In a 220 V AC grid for example a typical surge pulse has a level of above 1000 V. In an AC grid surges may occur between a phase line and the neutral line or between two phase lines. Such surges are known as symmetrical surges. If a surge occurs between a phase line and the protective earth line, the surge is known as an asymmetrical surge.
As such surge pulses may affect the function of the electrical devices connected to the grid or even damage or destroy them, the power supply of an electrical device usually is protected against such line surge voltages.
Protection against surge voltages can for example be achieved by connecting a protective component in series to the load in order to suppress the surges. This type of protective component for example includes: “Quarter-wave coaxial surge arrestor” and “Series Mode (SM) surge suppressor”. Another way to protect against surges is to connect a protective component in parallel to the load such that the overvoltage is diverted away from the load through the protective component. Examples for such components include: Metal oxide varistor (MOV), Transient voltage suppression (TVS) diode, Thyristor surge protection device (TSPD), Gas discharge tube (GDT), Selenium voltage suppressor or Carbon block spark gap overvoltage suppressor.
Generally such protective components are called SPDs (surge protection device) and they may also be combined to enhance the protection.
Which or which combination of SPDs is best suited in a particular application depends on the circumstances and the specification of the application and may be chosen accordingly.
In a low voltage AC grid such as an industrial, commercial or household electrical distribution system providing a one or multiphase AC supply network MOVs are the most widely used type of SPD, because they are low cost and effective. Such AC supply networks typically have a phase-to-neutral voltage of about 100 V to 300 V.
Document U.S. Pat. No. 8,345,400 B2 (Lutron Electronics) discloses such a surge suppression circuit having an MOV 222 connected in parallel between the phase and the neutral line to clamp the magnitude of a surge voltage to a predefined level.
Document U.S. Pat. No. 7,940,506 B2 (Dollar Energy Group) discloses another surge suppression apparatus. Again, MOVs are connected in parallel between the neutral and phase lines where the power supply is connected to the grid. Additional surge sense devices 106, 106′ such as fuses or thermal sensing devices are provided within the neutral and phase line to interrupt these lines if the current flowing through these lines becomes too high.
Document U.S. Pat. No. 6,118,639 (Goldstein) discloses a fast acting disconnect system with a transient voltage surge suppressor. The system includes a capacitor which, in case of an overvoltage, is charged by a charging circuit that uses the energy of the overvoltage to charge the capacitor. If the capacitor voltage reaches a predetermined level the energy stored in the capacitor is used to operate a disconnect switch for disconnecting the device from the grid. Accordingly, the whole device is disconnected wherefore the load cannot be operated any more.
Arranging an SPD such as for example a MOV in parallel to the load is an easy to implement solution. However, when choosing for example a varistor for a particular application, its voltage value is usually determined as about 25% higher than the maximum nominal operating voltage of the device to be protected. This is done to avoid that the SPD is activated in cases where this is not necessary. Accordingly, depending on the surge current the supply voltage is clamped by the SPD at a comparably high level. The level of this clamp voltage may be too high for the downstream circuit such that the downstream circuit may be damaged.